1. Field of the Invention
The present invention relates to a plasma generating apparatus and more particularly to a plasma generating apparatus for performing processings such as reforming, etching, ashing, cleaning and thin film formation at the surfaces of a semiconductor substrate, a liquid crystal glass substrate, an organic material, a metallic material or the like by using generated plasma.
2. Description of the Background Art
Plasma generating apparatuses for generating plasma by the electromagnetic energy of surface waves which are excited and propagated at a boundary of plasma and a surface of a dielectric used for introducing microwaves into a vacuum vessel have been used as a plasma generating apparatus using microwaves. A conventional plasma generating apparatus will be described below with reference to FIGS. 21-24.
As shown in FIG. 21, a surface wave plasma generating apparatus mainly used as a conventional plasma generating apparatus includes a vacuum vessel 101, a microwave oscillator 102, a driving power supply 103, a waveguide 105 and a dielectric plate 120. Vacuum vessel 101 has a gas supply port 101a for supplying a gas for discharging electrons (discharge gas) and a gas evacuation port 101b for evacuating the discharge gas which are provided in the vessel. A substrate 107 for various processings is placed on the lower side of the interior of vacuum vessel 101. on the upper side of the interior of vacuum vessel 101 are provided with a slit 106 and a dielectric plate 120 placed immediately thereunder and formed of a dielectric material. Dielectric plate 120 is connected to waveguide 105. Waveguide 105 is connected to microwave oscillator 102. Further, microwave oscillator 102 is connected to driving power supply 103.
The operation of the conventional surface wave plasma generating apparatus having the structure above will be described. First, a high vacuum pump (not shown) such as a roughing pump and a turbo-molecular pump connected to evacuation port 101b evacuates vacuum vessel 101 to a high vacuum, and a discharge gas such as argon, hydrogen, oxygen, chloride, carbon tetrafluoride and silane is supplied through gas supply port 101a. Thus, the interior of vacuum vessel 101 comes to have a prescribed pressure by the discharge gas. Then, microwave oscillator 102 oscillates microwaves by driving of driving power supply 103. The microwaves are emitted to waveguide 105. The microwaves pass through waveguide 105 and they are emitted from a microwave transmission circuit through slit 106. The emitted microwaves pass through dielectric plate 120 located on an upper surface of vacuum vessel 101 and they are introduced into vacuum vessel 101. Accordingly, plasma 108 is generated inside vacuum vessel 101.
When the density of plasma 108 increases after generation of plasma 108, the microwaves cannot progress into plasma 108. Therefore, they become surface waves generated on a surface of plasma 108 and they are guided in this form. The surface waves propagate along a boundary between dielectric plate 120 and plasma 108. The microwaves are absorbed by plasma 108 being propagated. As a result, in the vicinity of the surface of dielectric plate 120, electrons are accelerated by the vibrational electric field of the surface waves, attaining a high-energy state. The generated plasma 108 of high density is thus dispersed.
However, since dielectric plate 120 is located only on the upper side of vacuum vessel 101 in the conventional plasma generating apparatus as shown in FIG. 21, there caused a difference in microwave introduction between the portions near and remote from dielectric plate 120. It causes electrons and ions in plasma 108 to recombine together when dispersed. Thus, the distribution of the density of plasma 108 is made non-uniform in vacuum vessel 101. As a result, a processing of substrate 107 is also made non-uniform. When vacuum vessel 101 is to be enlarged as the diameter of substrate 107 becomes larger, electrons and ions in plasma 108 are also easily recombined together, the distribution of plasma 108 cannot be kept uniform, and the processing of substrate 107 tends to be non-uniform. This may affect the function of a semiconductor device, preventing the larger diameter of substrate 107.
In order to solve the problems above, the inventors proposed a plasma generating apparatus in which a dielectric tube 110 is inserted into vacuum vessel 101, as shown in FIGS. 22 and 23, as described in the co-pending application of U.S. Ser. No. 09/031,706, filed Feb. 27, 1998 now U.S. Pat. No. 6,054,016.
In the plasma generating apparatus, the vacuum vessel is of a column shape and dielectric tube 110 is arranged in the height direction. This allows uniform introduction of the microwaves in the height direction. The microwaves and a multi-cusp magnetic field 113 generated from magnets 111 shown in FIG. 24 serve to generate and maintain the plasma. It is therefore possible to allow a sufficiently uniform distribution of plasma 108 in the height direction of the column-shaped vacuum vessel.
However, since the apparatus shown in FIG. 22 has only one antenna 109 and one dielectric tube 110, microwave energy hardly reaches near a wall of vacuum vessel 101. Plasma 108 tends to disappear near the wall of vacuum vessel 101. Therefore, the distribution of plasma 108 in the diameter direction of column-shaped vacuum vessel 101 is not sufficiently uniform, and plasma 108 which is completely uniform cannot be generated.